(geology) Of, or relating to the geologic eon from about 3,800 to 2,500 million years ago; comprises the Eoarchean, Paleoarchean, Mesoarchean and Neoarchean eras; marked by an atmosphere with little oxygen, the formation of the first continents and oceans and the emergence of simple life.

(geology) The eon from 2,500 Ma to 4,000 Ma.

(biology) Any organism of the domain Archaea.

plural of archaean